1. Field of the Invention
The Present invention relates to a drive circuit for electro-absorption modulator and an optical transmitter using the same. More particularly, it relates to a drive circuit which can stabilize a bias voltage in the optical modulator and prevent degradation in waveform to realize long distance transmission, and an optical transmitter using the same.
1. Description of the Related Art
An external optical modulation system has been developed to realize a super high speed optical transmission system on 1.55 .mu.m wavelengths where transmission loss can be reduced.
An electro-absorption modulator has been proposed as an external optical modulator which can be driven with low power and is suitable to reduce the size of the modulator. The electro-absorption modulator absorbs a carrier light according to an applied voltage to generate an intensity-modulated optical signal.
FIG. 7 shows a structural block diagram of an optical transmitter employing the above-described electro-absorption modulator. In FIG. 7, a laser drive circuit 1 is formed of a laser light source 10 of which optical power is constantly controlled by an automatic power controller 11 and temperature is constantly controlled by an automatic temperature controller 12.
A carrier light radiated from laser light source 10 in laser drive circuit 1 is inputted to an optical modulator 2 formed of electro-absorption modulator EA. A drive circuit 3 supplies a driving voltage corresponding to an input signal Data to electro-absorption modulator EA in optical modulator 2.
Electro-absorption optical modulator EA absorbs the carrier light radiated from laser light source 10, i.e., it modulates the carrier light radiated from laser light source 10, according to the level of the driving voltage supplied from the drive circuit 3.
In this example, optical modulator 2 and laser light source 10 (LD) can be illustrated by an equivalent circuit shown in FIG. 8. Optical modulator 2 is formed by connecting electro-absorption modulator EA with a resistor RL in parallel. The applying voltage Vm at the anode of electro-absorption modulator EA is generated by flowing a current to resistor RL. Further, laser light source 10 is illustrated by a laser diode LD.
An optical output characteristic of electro-absorption modulator EA for the applying voltage Vm can be expressed by an equation approximated to exp, as shown in FIG. 9. It is apparent from the characteristic shown in FIG. 9 that optical output power P becomes maximum, when the applying voltage Vm is zero, i.e., light absorption in electro-absorption modulator EA is not effected and optical power of the carrier light radiated from laser light source 10 is outputted as it is when the applying voltage Vm is zero.
On the contrary, when the applying voltage Vm becomes larger, the optical output power becomes smaller by the light absorption as constructed by a curve approximated to exp. That is, a rate for absorbing the carrier light radiated from laser light source 10 in electro-absorption modulator EA becomes larger.
Optical power radiated from laser light source 10 can be absorbed in electro-absorption modulator EA, as converted to an optical current. The optical current is called as Iph which varies according to the applying voltage Vm shown in FIG. 10.
The current flowing to electro-absorption modulator EA and resistor RL which form optical modulator 2 will be now considered in accompanying to FIGS. 11A, 11B and 11C.
In FIG. 11A, a current driven by a modulating signal, i.e., an input signal, is IP, an optical current flowing to electro-absorption modulator EA is Iph2, and a current flowing to resistor RL is IR.
(1) As shown in FIG. 11B, if Iph1=0 mA, EQU Vm=IR.times.RL=(Ip-Iph2).times.RL 1
For example, if Iph2=20 mA and RL=50.OMEGA. when the value of Vm should be -3V, EQU Vm=60 mA.times.50=(80 mA-20 mA).times.50.
Accordingly, it is required to have a capability for flowing an extra current of Iph2=20 mA on the current IP driven by the modulating signal.
(2) When Iph1&gt;0 mA, it can be understood that the carrier light can be absorbed to flow the optical current even if Vm=0V as shown in FIG. 10 as a characteristic of an optical modulator.
For example, in a circuit of FIG. 11A, when Vm=0V, Ip becomes 0 mA. However, if the condition of IP=0 mA is substituted into the equation 1, Vm can be expressed as: EQU Vm=(0-Iph1).times.RL=-Iph1.times.RL.noteq.0 2
where Vm is negative voltage.
Therefore, it becomes apparent from the above-described equation that the relation is inconsistency.
Actually, the optical current Iph1 is flowing to resister RL. Vm can be obtained by Iph1.times.RL, and therefore, it becomes positive voltage.
In here, a chirping parameter .alpha., which may determine a transmission characteristic depends on the voltage Vm, as shown in FIG. 11C. Fluctuation of the chirping parameter .alpha. becomes larger near at Vm=0V.
Therefore, when the condition becomes to Vm&gt;0V, the transmission characteristic can not be guaranteed because the fluctuation of the chirping parameter .alpha. becomes larger. Therefore, a circuit for drawing the optical current Iph1 is required as shown in FIG. 12.
In this case, Vm can be calculated as follows; EQU Vm=IR.times.RL=(Ip+IB-Iph2).times.RL={(Ip-Iph2+Iph1)+(IB-Iph1)}.times.RL3
As IB=Iph1 in the equation 3, Vm=0 when Ip=0 mA.
It is general to control pulse current Ip and bias current IB to be constant not as changed by temperature variation and a secular change. However, when the optical current characteristic fluctuates from I to III as shown in FIG. 12B, IB is not equal to Iph1, and therefore, Vm is changed by the equation 3.
As the result, the transmission characteristic can not be made constant as .alpha. parameter is changed. Further, when there is a dispersion on each characteristic of electro-absorption modulator EA in optical modulator 2 similarly to the above-described case, it is required to control each electro-absorption modulator, and therefore, plural steps of adjustment are required.